**1. Introduction**

62 Biogas

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The biogas process has long been a part of our biotechnical solutions for the handling of sewage sludge and waste. However, in many cases the existing process applications need to be optimized to improve the extent of biogas production as a part of the energy supply in a sustainable and viable society. Although the principles are well known, process disturbances and poor substrate utilization in existing biogas plants are common and are in many cases likely linked to changes in the substrate composition.

Changes in substrate composition can be done as a means to obtain a more efficient utilization of existing biogas facilities, which today treating mainly manure or sewage sludge. By bring in more energy rich residues and wastes a co-digestion process with higher biogas potential per m3 volatile solids (VS) can often be obtained. However, new and changing feedstocks may result in shift in viscosity of the process liquid and, hence, problems with inadequate mixing, break down of stirrers and foaming. These disturbances may seriously affect the degradation efficiency and, hence, also the gas-production per unit organic material digested. In turn, operational malfunctions will cause significant logistic problems and increased operational costs. Changes of the substrate profile for a biogas plant may also infer modifications of the downstream treatment of the digestate.

Together with high digestion efficiency, i.e. maximum methane formation per reactor volume and time, the economy of a biogas plant operation depends on the energy invested to run the process. A main part of the energy consumed during operation of continuous stirred tank reactors (CSTRs) is due to the mixing of the reactor material (Nordberg and Edström, 2005). The shear force needed is dependent on the viscosity of the reactor liquid, where increasing viscosity demands a higher energy input. Active stirring must be implemented in order to bring the microorganisms in contact with the new feedstock, to facilitate the upflow of gas bubbles and to maintain an even temperature distribution in the digester. Up to 90% of biogas CSTR plants use mechanical stirring equipment (Weiland *et al.*, 2010).

In this context the rheological status of the reactor liquid as well as of the residual digestate are important for process mixing design and dimensioning. In addition experiences on rhelogical characterisation of sewage sludge revealing their dependence on the suspended

When measuring the dynamic viscosity, the fluid is subjected to a force impact caused by moving a body in the fluid. Resistance to this movement provides a measure of fluid viscosity. The dynamic viscosity can be measured using a rotation rheometer. The device consists of an external fixed cylinder with known radius and an internal cylinder or spindle with known radius and height. The space between the two cylinders is filled with the fluid

0 100 200 300 400 500 600 700 **1/s** 800

**.**

**Shear Rate** 

Fig. 1. Rheogram – flow curve of glycerol (▲) at 20 °C with a linear relationship between

Limit viscosity (lim) corresponds to the viscosity of a fluid at the maximum dispersion of the aggregates under the effect of the shear rate (Tixier & Guibad, 2003). The limit viscosity is estimated through the rheogram, when the dynamic viscosity becomes linear and constant. This parameter has been shown to be of great value when studying the rheological characteristics of sludge, since it determines the level of influence of important factors such as the total solids fraction (TS; Lotito *et al*., 1997). TS (%) and volatile solids (VS, % of TS) are parameters measured in the biogas process in order to control the amount of solids that may be transformed to methane. Also, Pevere and Guibad (2005) reported that the limit viscosity was sensitive to the physicochemical characteristics of granular sludge, i.e. it was influenced

shear stress (; Pa) and shear rate (; s-1), representing a Newtonian liquid.

by changes in the particle size or the zeta potential.

subjected to dynamic viscosity analysis.

**2.2 Limit viscosity** 

130 **Pa**

solid concentration and on the characteristics of the organic material as well as on the interactions between particles and molecules in the solution (Foster, 2002). Therefore, this type of characterisation can be important in process monitoring and control.

The aim of this chapter is to briefly introduce the area of rheology and to present important parameters for rheological characterization of biogas reactor fluids. Examples are given from investigations on such parameters for lab-scale reactors digesting different substrates.
